We have found that the decline in aging mice of the in vitro generation of TNP-specific killer T cells reflects a loss of IL-2 secreting, Lyt-2 minus helper cells. We are developing a limiting dilution assay for IL-2 producing cells, an in vitro method for producing antigen-specific primed helper cells, and an in vivo technique for quantitating the ability of spleen T cell subsets to generate mature, antigen-responsive helpers. The effects of thymic hormone analogues and soluble mediators produced by thymic-processed cells on the progress of T cell development will be studied to learn more about how these developmental events break down in the aging mouse.